Those of us who have watched and kept tadpoles will have noticed that sometimes they come to the surface to take in air. A recent paper shows that very young and therefore very small tadpoles cannot break through the surface tension of the water in which they live. However, they still manage to take in air by a process the authors call ‘bubble-sucking’.
Even three days after hatching and only 3 mm long tadpoles have been found to fill their lungs. High-speed video showed what was happening. In five species of frog from North America and in the much-studied African Clawed Frog, Xenopus laevis the authors found:
…mouth attachment to the water’s undersurface, the surface drawn into the mouth by suction, a bubble ‘pinched off’ within the mouth, then compressed and forced into the lungs.
As tadpoles grow, they gain the size and strength to breach the surface of the water to take in air. However, larger tadpoles of one species studied (Grey Treefrog, Hyla versicolor) continued to use bubble-sucking exclusively while those of others used both methods until metamorphosis.
The authors recorded a similar of taking in air in salamander larvae which have external gills.
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| From Schwenk & Phillips 2020 |
It would be easy to assume that tadpoles take in air in order to extract the oxygen it contains. Tadpoles would normally be expected to respire through their gills and skin. With low concentrations of oxygen in the water, hot or fetid, for example, the selective advantage of being able to breathe air is obvious. However, the presence of air in the lungs is related to another function in these aquatic organisms—the control of buoyancy. I am surprised that the authors in their discursive account chose to concentrate on a presumed respiratory function when there has been a considerable amount published on air in the lungs in relation to the control of buoyancy. The authors themselves noted that in at least one of the species studied the lungs at 3 days after hatching are poorly vascularised and do not acquire a rich blood supply typical of a site of gaseous exchange until later in development.
Tadpoles, by having the ability to fill their lungs with air using the newly discovered method of ‘bubble-sucking’ to overcome the surface tension of water, thus have the opportunity to use that air for the control of buoyancy and as a source of oxygen, depending on their stage of development and the degree of oxygenation of the water in which they live.
And I am still fascinated by tadpoles despite the strictures of my grandfather that I would never be able to make a living by studying them.
Schwenk K, Phillips JR. 2020. Circumventing surface tension: tadpoles suck bubbles to breathe air. Proceedings of the Royal Society B 287: 20192704. http://dx.doi.org/10.1098/rspb.2019.2704
see also
Gee JH, Rondeau SL. 2012. Strategies used By tadpoles to optimize buoyancy in different habitats. Herpetologica 68, 3-13 doi.org/10.1655/HERPETOLOGICA-D-10-00023.1
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